Citigroup admitted Wednesday that an attack on its website allowed hackers to view customers' names, account numbers and contact information such as email addresses for about 210,000 of its cardholders in North America.

Citigroup did not say how the website, Citi Account Online -- which is used by its customers to manage their cards -- was compromised but that the discovery came through its "routine monitoring." The bank discovered the breach, which was first reported in Thursday's Financial Times, early last month.